Almost There; almost there!
Somwthing that I think everyone needs to know about insect biology.
The reason thri-kreen are immune to sleep effects, and don't actually sleep: No eyelids. They can't close their eyes.
The reason they cannot wear armor, or swim: Insects and arachnids do not have lungs. Because of their chitonous bodies, their exoskeleton, they can not expand their chests to draw in breath. Instead, they breath through holes located in their torsos or their abdomens (I believe the AD&D splat book Thri-kreen of Athas places their spericles under both pair of their arms). These holses have no "lips", or any other means to close. If you have ever seen a still insect, it is likely dead. When you a preying mantis sitting still, it is swaying slightly. That movement is how it pumps air in and out of its sphericles, and throughout its body. It stops moving too long, it suffocates. Covering the sphericles with armor or clothing would make it very difficult for them to breathe. If they fall into water, they have no way to hold their breath, and their movements to save themselves actually cause them to drown faster. It also explains why aerosol poisons are so effective against arachnids and insects.
They don't explain it in game, but these would be the reasons the kreen don't sleep, and don't wear armor, and don't swim.